1. Field of the Invention
This invention relates to E-beam lithography and particularly to a correction system for the elimination of the effects of variations of the height of the wafer or mask surfaces. Such variations tend to degrade focus (image quality) and deflection magnification and rotation (unregistered writing.)
2. Description of Related Art
Commonly assigned U.S. Pat. No. 4,468,565 of Blair et al for an "Automatic Focus and Deflection Correction in E-Beam System Using Optical Target Height Measurements" describes a prior art autofocus system. The specific part of the Blair et al system which has been improved upon with this invention is the autofocus electronic subsystem.
FIG. 1 illustrates the optical subsystem interface between the system optics and the electronic subsystem. This optical system basically incorporates "light lever" techniques adapted specifically to the particular conditions unique to an E-beam exposure tool. The criterion of E-beam use are satisfied by the optical system shown in FIG. 1.
The light source forms an image of the slit on the surface of the target. This slit image is reflected off the target through the optics shown in FIG. 1 to the light transducer i.e. a light sensor. This light sensor converts the light image into a video output. It is this video output which is used by the automatic-focus (autofocus) electronics to determine the height of the target. As shown in FIG. 1, variations of the z-position of the target surface 15 by plus or minus delta z causes the slit image 16 to move vertically to the position 9' from position 9. It is this correlation between the linear position of the video signal on the sensor to the change in height which allows for the measurement of the height of the target.
The Blair et al autofocus electronic subsystem uses, as an input, the typical video signal output of the diode array 28 in FIG. 3, which output is shown in FIG. 2. The output from the diode array 28 has a clipping level established by means of a comparator 37 to determine which of the diodes N1, N2, . . . Nn, (where there are n diodes) in the array 28, for example, diode N1, corresponds to the leading edge of the light beam and which diode, N2, corresponds to the trailing edge of the light beam. The electronic subsystem utilizes the output of the diode array, as shown in FIG. 2, and converts that output into signals needed for correcting the E-beam focus and deflection. Moreover, the electronic subsystem of FIG. 3 adjusts the sensor signal magnitude to compensate for varying conditions, such as, a varying reflectivity of the target surface.
Referring now to FIG. 3, the autofocus electronics subsystem of the prior art is described in more detail. The video output signal shown in FIG. 2, from the diode array 28 shown in FIG. 3 is initially fed to a difference amplifier 32 and then to a gain adjusting control resistor 33. A base line servo correction 36 is then added to the video signal at the summing junction 34 of the amplifier 35.
The baseline servo 36 is required to correct for DC offset drift in the amplifiers 32 as a result of large DC gain necessary to amplify the small diode signal and raise it to a level that can be processed by the comparator 37 in FIG. 3. The resulting video output signal 38 is then identical to that in FIG. 2 where the clipping level is used by a comparator 37 to determine which diode (N1) corresponds to the leading edge of the light beam and which diode (N2) corresponds to the trailing edge of the light beam shown in the output of FIG. 2.
The system of FIG. 3 utilizes logic circuits 39 receiving the comparator 37 output and secondly the output from the clock 40 to generate a count equal to the number of diodes from the beginning of the diode array 28 to the center of the light beam. An output section 41 receives the count together with a reset signal and a strobe signal.
The output section 41 performs multiple functions. First, it converts the serial count from the logic 39 into a parallel output word sent to a controller computer, not shown. Also, the output section employs a digital-to-analog converter to produce an analog output used to correct the E-beam focus, directly. These two outputs are indicated in FIG. 3.
The values of height which are transmitted to the computer and to the focus coil driver of the E-beam assembly are periodically updated when pulses occur on the strobe and reset lines.
FIG. 3 shows an input-to-logic 39 in the form of "computer" C-cycle. This input indicates that the E-beam system is in a table move mode and accordingly the system is at a point in operation where updating of the autofocus outputs can occur. In the table move mode, there is neither writing nor registration of the system but rather a physical adjustment is being made to the specimen which permits updating of the autofocus outputs. Accordingly, utilizing the C-cycle input to the logic section 39, updating occurs at a point in the processing where there is no substantive E-beam operation.
FIG. 3 shows an amplitude servo 42 receiving respective inputs from the clock and counters 40 and the base line servo 36. The amplitude servo 42 is utilized to compensate for different resists and for different reflectives due to different resists and different structures on the wafers. The amplitude servo 42 is also used to compensate for drift in the LED light output and amplifier gain drift. The amplitude servo is further used to measure the output of the strongest diode and determine whether it is of the proper amplitude. If the output is too low, the amplitude servo 42 increases the time between start pulses sent to diode array 28. An increase in time between start pulses allows the diodes in the array 28 to receive light for a longer period of time, and therefore have a larger signal when read out. Conversely, if the signal is too large, the time between start pulses is decreased, which results in a decrease in output amplitude. If the amplitude servo 42 does not detect a signal, then, an output is delivered to the output section 41 indicating that a wafer is not present. As a result, the output section switches to preprogrammed default values which are stored in thumbwheel switches 43.
As shown in FIG. 3, the output section 41 provides a digital output to the computer controller representing the measured height of the chip which is being written. The computer then uses this value to compute and apply corrections to the magnification and rotation of the magnetic deflection corresponding to the measured height during unregistered reading.
Also, as shown in FIG. 3, the output section 41 provides an analog signal which is used directly and automatically to correct the E-beam focus for different heights of each chip on the wafer.
The resolution attainable with the Blair et al autofocus electronic subsystem is limited by the signal processing schemes employed.
First, the signal processing scheme used to read the light transducer is unable to provide the signal-to-noise ratios needed to operate at this resolution.
Second, the method used in a subsystem of this invention to determine the height, which generates the focus, magnification, and rotation corrections, is theoretically not able to achieve a resolution beyond 1 micron. The subsystem determines the height of the target based on the leading and trailing edges of the video signal. The video signal is generated from the readout of discrete photodiodes in the sensor. Since the leading and trailing edges of the video signal are in discrete steps, there is no chance to interpolate between diodes. Because the distance between diodes corresponds to a height change of one micron, it is impossible to have a resolution higher than 1 micron.
Third, the video signal amplitude servo used in the Blair is unable to provide the accuracy needed to work at this resolution. Digital techniques were employed in the Blair et al amplitude servo which cause changes to be made in the amplitude of the sensor output every time the sensor is read out. These continuous sensor output amplitude variations make it impossible to work at a higher resolution.